ABSTRACT The ER/SR compartment in a cardiomyocyte is highly specialized for controlling calcium fluxing in excitation- contraction coupling (ECC), as well as for regulating protein synthesis and stress responsiveness to unfolded proteins. The traditional ER stress response involves sensing of calcium and unfolded or damaged proteins in the ER through 3 distinct pathways that initiate a cascade of signaling to alter protein synthesis and other features of cellular adaptation to stress. We recently identified thrombospondin 4 (TSP4) as a stress-inducible factor that resides for a period of time in the ER/SR before being secreted to the extracellular matrix (ECM), where it alters the ER stress response. The heart expresses TSP1, TSP2, and TSP4, each of which is dramatically up-regulated following injury or stress stimulation. Interestingly, TSP4 is only expressed in heart and skeletal muscle, and it appears to be of an entirely different functional subclass from TSP1 and TSP2. We have identified a novel function for TSP4 as a cardiac inducible protein that dramatically enhances the content and function of the ER/SR resulting in greater contractility, increased activity of the adaptive ER stress response, and protection from heart failure-inducing stimuli. Thus, we hypothesize that TSP4 is a novel adaptive stress-response factor that benefits ER/SR function to provide cardioprotection. In this project we will: 1) determine if TSP4 protects the heart from failure through adaptive ER stress response pathway engagement, 2) investigate the ER stress response factors that mediate TSP4-dependent cardio-protection, and 3) determine how and where TSP4 signals the adaptive ER stress response. We will use TSP4 transgenic and gene-targeted mice to investigate these 3 specific aims, as well as numerous transgenic models with altered ER stress signaling or protein aggregation-based cardiomyopathy. Extensive in vitro molecular approaches are also proposed to identify the mechanism whereby TSP4 coordinates the protective ER stress response and benefits the heart. Finally, numerous collaborations with the Kranias and Robbins lab's are proposed to determine how TSP4 affects calcium handling and the unfolded proteins response.